Continuously variable transmissions (CVTs) include a continuously variable unit, such as a belt and pulley mechanism. The effective ratio of the CVT is determined by the radius at which the belt engages the pulleys. In most instances, the ratio can be varied from an underdrive to an overdrive. CVTs typically include a first cone pulley pair on an input shaft as a primary pulley set and a second cone pulley pair on an output shaft as a secondary pulley set. Each cone pulley pair consists of a first axially stationary pulley and a second axially movable pulley. Between the cone pulley pairs rotates a belt, chain or torque transmission member which is wound around each cone pulley pair. The running radius of the torque-transmitting belt may be adjusted by adjustment of the cone pulley pairs. Adjustment of the cone pulley pairs, accordingly, adjusts the running radius of the pulley at the input shaft and output shaft, thereby adjusting the output/input speed ratio of the CVT.
In order to adjust the primary or secondary pulley set, the respective axially movable pulley is actuated with a pressure medium from a pressure source. The ratio of the CVT is changed by reducing or increasing the pressure acting on one of the sheave halves of one of the pulleys, generally the input pulley, while the pressure at the other pulley may be maintained substantially constant. The continuously variable unit requires a high pressure to ensure sufficient clamping forces for the belt and pulley mechanism, as slippage of the belt against the pulleys is often undesirable. The amount of clamping pressure required is a function of the input torque to the transmission and the ratio at which the variable transmission unit is operating. If the clamping pressure is low, there is a possibility of belt slippage.
The control pressure level required to engage the torque transmitting mechanisms is generally lower than the pressure required to control the CVT pulleys. The amount of pressure required in the torque transmitting mechanisms is essentially a function of torque being transmitted and size of the conventional clutch hardware, consisting of a movable piston and a clutch pack. If the control pressure is below the required value, slippage of the friction plates can occur, which will shorten the life of the torque transmitting mechanisms.
In order to increase the fuel economy of motor vehicles having conventional planetary gear automatic transmissions, it has been desirable to stop the engine during certain circumstances, such as when stopped at a red light or idling. However, after the engine has been shut down and has remained off for an extended period of time, the fluid generally tends to drain down from the passages into a transmission sump under the force of gravity. Upon engine restart, the transmission may take an appreciable amount of time to establish pressure before full transmission operation may resume. Such engine start/stop algorithms have typically not been used in CVT transmission systems due to the extra amount of time and fluid pressure that it would take to bring the CVT transmission up to the pressure that it needs to properly operate the pulleys without belt slippage.